The present invention relates to a device for operating at least two discharge lamps, having a current control device for controlling the current through the incandescent filaments. In particular, the present invention relates to electronic ballasts in which such a device is integrated. Operating discharge lamps comprises in this case both their starting and their being alight.
It is known to operate two discharge lamps with two load circuits. In this case the term load circuit refers to the load of a bridge that is used as an inverter to operate a discharge lamp. Each load circuit has a dedicated preheating arrangement for the respective lamp. Furthermore, according to the internal prior art it is possible to operate two lamps in one load circuit. Here, the primary coil of a heating transformer of a series circuit of two lamps is connected in parallel and the secondary coil of the heating transformer is connected between the two lamps. Furthermore, it is possible to heat all the filaments of the lamps by transformer via secondary windings, the primary winding being situated in a section of the bridge suitable for the application.
It is relatively complicated to implement the load circuits in terms of circuitry, since electronic control circuits with relay or transistor switches are required for a defined, sequential starting and subsequent joint operation of the lamps. By contrast, relatively favorable control circuits that use only passive components for controlling the preheating exist for the purpose of operating individual lamps. The essential constituent of such circuits is a heat-sensitive resistor with a positive temperature coefficient.
A bridge circuit with a relevant load circuit is illustrated in FIG. 1. The bridge is implemented for the purpose of inversion as a half bridge with two switching elements 1 and 2 and two capacitors 3 and 4. The load circuit 5 in the bridge comprises a coil 6 in series with a lamp 7 which is connected in parallel both with a resonance capacitor 8 and with a heat-sensitive resistor 9.
The mode of operation of the circuit illustrated in FIG. 1 may be explained as follows. By actuating the switches 1 and 2 suitably, an AC voltage for the load circuit 5 is generated in the center tap of the bridge from the DC voltage. The frequency of the AC voltage is advantageously in the region of the resonant frequency of the coil 6 and the capacitor 8 for the ignition process of the lamp. Before the ignition, as PTC thermistor the resistor 9 with a positive temperature coefficient (PTC) detunes the series resonant circuit 6, 8 in such a way that the required ignition voltage across the lamp 7 or the capacitor 8 is not reached. However, the current is already flowing through the incandescent filaments 10 and 11 of the lamp 7 such that they are preheated for the ignition process. In the meantime, current is likewise flowing through the PTC thermistor 9, which it heats in this preheating phase. Its resistance rises in the process, and so the detuning of the series resonant circuit, 6, 8 is correspondingly reduced such that the ignition voltage across the lamp 7 can be reached. The PTC thermistor 9 is designed in this case such that it carries a sufficient quantity of current even after ignition in order to remain highly resistant so that the resonance can be maintained at an appropriate level of quality.
For the sake of clarity, the load circuit 5 is illustrated in FIG. 2a without the coil 6. FIG. 2b shows a variant of the load circuit of FIG. 2a. Connected in series with the PTC thermistor 9 is a series capacitor 12 which has the effect that the detuning of the resonant circuit by the PTC thermistor 9 is not so marked as in the case of the circuit of FIG. 2a. This means that in this case the ignition voltage is reached more quickly and the lamp is ignited more rapidly as a consequence thereof.
A further variant of the load circuits that are illustrated in FIGS. 2a and 2b is reproduced in 2c. In this case, the series capacitor 12 is chiefly active in the cold state of the PTC thermistor 9, whereas the series circuit of the two capacitors 8 and 9 is only active in the warm state of the PTC thermistor 9, that is to say during the operation and ignition of the lamp.
The object of the present invention consists in proposing a cost effective preheating circuit for operating two lamps.
According to the invention, this object is achieved by means of a device for operating at least two discharge lamps having a first contact device for electrically connecting a first discharge lamp, which has two first incandescent filaments, a second contact device for electrically connecting a second discharge lamp, which has two second incandescent filaments and a current control device for controlling the current through the two first and two second incandescent filaments, wherein terminals of the first contact device for one of the first incandescent filaments are connected to terminals of the second contact device for one of the second incandescent filaments together with a secondary winding of a transformer device in the circuit, and wherein one terminal each of the first and second contact device for the respective other one of the first and second incandescent filaments are interconnected, with the interposition of the current control device, in series with the primary winding of the transformer device.
The advantage of the inventive circuit resides in that by comparison with the preheating circuit for one lamp the additional outlay for preheating a second lamp is present only in one component, specifically a transformer. Given suitable dimensioning, the transformer ensures that all the incandescent filaments of the discharge lamps are heated simultaneously and with approximately the same power.
In one advantageous refinement, a resonance capacitor is connected in parallel with the inventive device, that is to say between the remaining terminals of the two contact devices. The two lamps can thereby be operated with the aid of a resonant circuit.
The current control device advantageously comprises a PTC thermistor with a positive temperature coefficient. This component permits a relatively simple and cost-effective control of the preheating for the lamps. Instead of the PTC thermistors, the current control device can comprise a transistor. It is possible thereby to control the preheating in a more targeted but also more complicated way.
A series capacitor can be connected in series with the current control device; it has the effect that the resonant circuit is detuned less overall, and the lamps are ignited earlier by a corresponding increase in current.
A sequential starting capacitor can be provided in parallel with the first and/or second contact device; it can be used advantageously to control the sequential starting sequence in the case of at least two lamps. Consequently, it is possible to achieve sequential starting in order to avoid very high ignition currents/voltages being reached, said starting permitting the use of components which cannot be too highly loaded and are therefore more cost-effective.
Also, the device preferably can be connected to an inductor with the aid of which the device can be operated in resonance. The device can thereby be driven by a single inverter for the purpose of operating two or more lamps.
The inventive device is advantageously integrated in an electronic ballast for fluorescent lamps. It is thereby possible to operate two or more lamps with the aid of one ballast.